1. Field of the Invention
The present invention relates to an electrical circuit, and particularly, to an electronic data and signal processing control system, and more particularly, to a signal conditioning unit which detects high impedance input conditions and maintains accurate gain and input impedance over a wide range of operating parameters.
2. Description of Related Art
Signal conditioning circuits are often used as an interface in a signal conditioning unit to convert a differential input signal received from a data source into a more usable output signal. Signal conditioning circuits may be used in conjunction with sensors to receive a sensor input signal and convert the input signal into an output voltage usable by a control system. For example, in order to maintain the quality of air, it is desirable to control internal combustion engine emissions. A signal conditioning circuit may be used in conjunction with an oxygen sensor to produce an output voltage that is related to the oxygen partial pressure in an atmosphere being sensed by the oxygen sensor. The output voltage is received by a signal processing control system to control the fuel-air mixture supplied to the engine which regulates engine performance and reduces undesirable emissions.
The automotive environment introduces a number of severe requirements for the sensor-signal conditioning combination. First, a single power supply is used, usually the automotive battery, which has one terminal connected directly to the vehicle frame. While the oxygen sensor is also connected to the vehicle frame or to the engine block, its location is normally remote from the signal conditioning circuit so that the two frame locations or frame and engine block locations can be operating at substantially different potentials. For example, the oxygen sensor ground can be operating at a DC potential of .+-.2.0 volts with respect to the signal conditioning circuit ground. This represents a significant common mode range. Second, the signal conditioning circuit must cope with an oxygen sensor that produces a fractional volt output which varies with oxygen content and has an internal resistance that varies over several orders of magnitude as a function of operating temperature. The signal conditioning circuit should produce a nominal output when the sensor is cold and then produce an oxygen related output as the oxygen sensor warms up during use. Third, the signal conditioning circuit should quickly detect high impedance conditions at the sensor signal conditioning circuit sensor inputs in order to facilitate accurate and prompt response by the receiving control system circuit. Fourth, current flow must be accurately controlled to meet strict requirements involving, for example, sourcing current to the sensor. Fifth, the signal conditioning circuit should require minimal cost expenditures, minimal testing, and occupy minimal space using either discreet and/or integrated components.
FIG. 1 is a schematic diagram of an interface circuit 100 fabricated as an integrated circuit and utilized as a signal conditioning circuit in an automotive environment to interface between an oxygen sensor and a fuel-air mixture control system. The interface circuit 100 is a LM1964D circuit manufactured by National Semiconductor, a Santa Clara, Calif. company. To provide a common mode operating range, the interface circuit 100 uses input diodes to level shift the input signal. As a result, the acceptable common mode range of interface circuit 100 varies with temperature because the diode voltage decreases when the operating temperature of the interface circuit 100 increases and is only about .+-.1 V.
In an automotive application, an oxygen sensor 124 represented by a variable resistor 114 and a voltage source 116 is connected via filters 118 and 120 across the non-inverting input 110 and the inverting input 112 of interface circuit 100. In order to detect open circuit conditions at the input of interface circuit 100, an open circuit detection circuit 104 is also connected across the non-inverting input 110 and the inverting input 112 and provides a bias voltage with the 1.2 Mohm resistor 122 conducting a current of only a few nanoamps. When the series impedance of the oxygen sensor 124 is low compared to resistor 122, as is the case in normal operation, the oxygen sensor 124 sinks the open circuit detection circuit 104 current, and the open circuit detection circuit 104 only minimally affects the interface circuit 100 output voltage. However, when a lead in the external circuit between non-inverting input 110 and inverting input 112 is broken and causes an open circuit condition or when the oxygen sensor 124 is cold and presents a high impedance, the open circuit detection circuit 104 provides the input current to drive the interface circuit 100 output to a predetermined value to establish a default fuel-air mixture. However, because the small current on the order of a few nanoamps is used in the open circuit detection circuit 104 to facilitate normal oxygen sensor 124 operation, the reaction time of interface circuit 100 to oxygen sensor 124 open circuit conditions extends over a finite period of time, t.sub.oc, and is too lengthy for more stringent emissions control specifications.
Additionally, as emissions control specifications become more stringent, a signal conditioning circuit must be able to comply by reacting more quickly and accurately not only to open circuit conditions but also to changing sensor inputs. A microcontroller (not shown) relies in part on the accuracy of the interface circuit 100 input impedance and precise gain characteristics to provide accurate and responsive control signals to other systems such as the fuel-air mixture controller (not shown). The interface circuit 100 incorporates zener diodes in circuits 102, 106, and 108 which are trimmed during testing to provide input impedance and gain characteristics. However, zener trimming is associated with parameter tolerances of such magnitude that providing the precision operating parameters for interface circuit 100 necessary to meet ever more stringent emissions control specifications will be increasingly non-trivial and costly to achieve especially over a large production run of fabricated interface circuits.
Another disadvantage of zener trimming on the interface circuit 100 involves the costs associated with testing and fabrication. Testing of the interface circuit 100 is performed by a test system at the wafer level by measuring circuit errors in response to applied test signals. In order to correct the measured errors, the test system short circuits the zener diodes. The costs associated with zener trimming include test time to measure errors, trim the zeners, and repeat the test process until desirable circuit characteristics are obtained. Furthermore, the zener trimming requires allocating valuable circuit area to diodes which potentially may be short circuited.